1. Field of the Invention
The invention relates to a surface controlled subsurface safety valve (SCSSV) for a sub-terranean well and, more particularly, to a safety valve utilizing an electrical actuation mechanism controlled from the surface or by a downhole intelligent controller.
2. Prior Art
Employment of a downhole safety valve is well known for subterranean oil and gas wells. Such valves, which can comprise a plug or pocket type, a sleeve valve, a flapper valve or ball valve, are normally positioned downhole to close the bore of the tubing string leading from one or more production zones to the well surface. Safety valves of this type are normally biased to a fail safe condition whereby any significant reduction in the opening force acting upon a valve will allow a pre-energized arrangement such as a spring to close the valve.
Commonly, downhole safety valves are actuated by hydraulics. A hydraulic system is connected to a piston arrangement; pressure from the surface via a small diameter control line is directed upon the piston which, in turn, moves a flow tube past a flapper valve thereby opening the flapper valve. In this position the flapper is essentially locked in the open position by the presence of the flow tube. A spring is generally placed in contact with the flow tube and with a non-moveable housing so that when the flow tube is urged downhole it is against the bias of the spring, thus energizing the spring for closing of the valve should the opening impetus caused by the hydraulic pressure from above be lost or reduced. While these valves are highly effective in the field, they do have drawbacks. One such drawback is that since these valves are installed many thousands of feet below the earth's surface, thus necessitating many thousands of feet of hydraulic control line, the hydrostatic pressure of the control line is sufficient to render the closing of the safety valve a slow process. In order to close the valve, the spring must lift the hydraulic column all of the fluid contained in the piston cylinder back into the control line by forcing, for instance, six thousand feet of fluid in that control line uphole. This requires a strong closure spring to lift the hydraulic fluid column. Necessarily, the control line is susceptible to damage during the running process and the joints in the control line may develop leaks over time. Such a leak would indicate a reduction in the opening impetus and the fail safe feature of the valve would close the same. Loss of integrity of the control line, in general, requires that the entire tubing string be pulled from the well and necessary repairs made.
More recently electric actuation of downhole safety valves has become of interest. Electrically operated safety valves are becoming increasingly popular with the introduction of intelligent downhole systems such as those disclosed in U.S. patent application Ser. No. 08/386,504 now U.S. Pat. No. 5,706,896 assigned to the assignee hereof and incorporated herein by reference. Downhole intelligence allows an electrically actuated downhole safety valve to receive commands from the surface or downhole and thereby operate either completely automatically or with input if desired.
One of the drawbacks associated with the use of solenoid actuated downhole safety valves arises from the use of the solenoid itself to directly open the valve. Opening valves of larger sizes requires a reasonably long throw. Solenoids, however, generally operate on throws shorter than that necessary. In many cases the throw of the solenoid is not sufficient to completely open the safety valve. This impedes flow of production fluid and risks damage to the safety valve due to the bending moment on the pivot point of the valve caused by production flow. To remedy the drawback, either a larger solenoid or various levering arrangements have been employed with some success. There is still a need, however, for improved methods of electrically actuating the downhole safety valve.